Trump is already walking back his most important promise on the economy

President Donald Trump rose to power in a surprise electoral victory after promising a disaffected electorate that he could more than double the US's rate of economic growth to 4%.

Barely a month into Trump's presidency, his advisers have already downgraded that rather elevated forecast.

Steve Mnuchin, Trump's recently confirmed Treasury secretary who is a former banker at Goldman Sachs, has been making the rounds talking up plans for a round of tax cuts. At the same time, he has substantially downgraded the White House's goal for economic growth — to just 3%.

The White House website still boasts that "to get the economy back on track, President Trump has outlined a bold plan to create 25 million new American jobs in the next decade and return to 4% annual economic growth."

But in an interview with CNBC, Mnuchin was signing a much softer tune. "We believe we can get back to sustainable growth of 3% or more," he said.

To put the difference in perspective, a full percentage point of growth could equate to millions of new jobs, so the margin of error is hardly trivial.

Investors might be reminded of a recent pattern at the Federal Reserve, which has consistently overestimated the likely strength of US gross-domestic-product growth at the start of each of the past several years only to turn tail as conditions worsened or unexpected shocks hit the economy.